refactor(speaking): drop AI voice-bar from DetailedReport

After removing the AI play button and duration in Task 7, the AI card's
voice-bar contained only a decorative waveform — orphaned content that
adds no information. Gate the entire voice-bar on student role and drop
the now-dead role-conditional classes (bar-ai/cw-ai/cd-ai branches).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/views/Editor/EnglishSpeaking/preview/DetailedReport.vue

@@ -28,10 +28,9 @@
           <!-- 卡片主体 -->
           <div class="card-body" :class="{ 'body-student': sentence.role === 'student' }">
             <div class="card-left">
-              <!-- 语音条 -->
-              <div class="card-voice-bar" :class="sentence.role === 'ai' ? 'bar-ai' : 'bar-student'">
+              <!-- 语音条（仅学生卡片，AI 无 audio_url 故纯文字呈现） -->
+              <div v-if="sentence.role === 'student'" class="card-voice-bar bar-student">
                 <button
-                  v-if="sentence.role === 'student'"
                   class="card-play-btn play-student"
                   :disabled="!sentence.audioUrl"
                   @click.stop="handleSentencePlay(sentence)"
@@ -75,15 +74,11 @@
                   <div
                     v-for="i in 8"
                     :key="i"
-                    class="card-wave-bar"
-                    :class="sentence.role === 'ai' ? 'cw-ai' : 'cw-student'"
+                    class="card-wave-bar cw-student"
                     :style="{ height: `${Math.sin(i * 0.6) * 4 + 3}px` }"
                   />
                 </div>
-                <span
-                  v-if="sentence.role === 'student'"
-                  class="card-duration cd-student"
-                >
+                <span class="card-duration cd-student">
                   {{ formatDuration(sentence.audioDuration) }}
                 </span>
               </div>